1. Field of the Invention
This invention relates to anodes used in the electrolytic extraction of aluminum metal from alumina (aluminum oxide) ore. In particular, it relates to inert anodes with noble-metal coatings.
2. Related Art
The most widely used process in commercial aluminum production is the Hall-Heroult process which utilizes an electrolytic furnace in which the electrolyte is a bath of fused fluorides and cryolite at typically 900 deg C. The cathode is carbon which lines the vertical walls and bottom of the furnace. The anode consists of vertical carbon bars which dip into the bath.
Powdered alumina ore is dropped into the bath from above and an electric current is passed through the bath via cathode and anode. The resulting electrolysis separates out pure aluminum metal at the cathode (where it is periodically tapped), and oxygen at the anode which it attacks, consuming the carbon anode to form carbon monoxide and carbon dioxide. The anode consumption rate is roughly equal to the aluminum production rate.
To avoid the continuous replacement of carbon anodes and the emission of greenhouse gases such as carbon dioxide, a search has been undertaken for an inert non-carbon anode which can withstand the corrosivity of the high-temperature salt bath, is not attacked and consumed by the oxygen, and yet has high electrical conductivity.
One class of materials considered has been advanced ceramics such as refractories, monolithic ceramics, ceramic composites, and coatings.
Two comprehensive reports on the subject have been published:
1. xe2x80x9cInert Anode Roadmapxe2x80x94A Framework of Technology Developmentxe2x80x9d published by Energetics, Inc., Columbia, Md. (February 1998);
2. xe2x80x9cReport of the ASME""s Technical Working Group on Inert Anode Technologiesxe2x80x9d published by American Society of Mechanical Engineers (July 1999).
Ref.1 established essential performance targets for inert anodes, such as: low erosion rate, high electrical conductivity, low polarization voltage, good structural properties, stability in high-temperature oxygen, good metal quality, and environmental and safety acceptability. After reviewing the state of the art, Ref.1 states that xe2x80x9ca viable material for fabricating the anodes has not yet been demonstratedxe2x80x9d.
Ref.2 xe2x80x9cprovides a broad assessment of,open literature and patents that exist in the area of inert anodes . . . xe2x80x9d. A patent search uncovered more than 119 patents going back to 1985 and a further 229 patents going back to 1945. Progress in inert anode materials was found, such as cermets of nickel-iron-copper and self-passivating metallic alloys. However, for practical applications xe2x80x9cto date, no fully acceptable inert anode materials have been revealedxe2x80x9d. Recommendations for future RandD resulted in a first priority for metals protected with coatings. One of the industry experts doubted that micron-thin noble-metal coatings would remain intact on metallic substrates.
Contrary claims have been made in the noble-metal coating field for the SCX low-temperature sputter coating process which is computer-aided and proprietary to Englehard-CLAL, Carteret, N.J. As described in the article xe2x80x9cUnique Coating Technology Enables Co-Deposition of Noble Metalsxe2x80x9d, Industrial Heating (October 1997), micron-thin platinum coatings were successfully deposited on metal wires of diameters as small as 10 mil (and even smaller) by this process.
In view of the related art described above, the following desirable characteristics are set forth as objects of a viable inert anode for electrolytic aluminum production:
1. Has high electrical conductivity, above that of carbon;
2. Generates anodic oxygen rather than carbon dioxide;
3. Has inert surface, making the anode non-consumable;
4. Has catalytic surface to promote dissociation of oxides in the electrolysis;
5. Made of material which remains solid at 900 deg C., considerably above the temperature of the electrolysis.
6. Has surface which resists corrosion when exposed to fused fluoride salts and molten aluminum metal;
7. Has modular geometry expandable to fit large furnaces; and
8. With production costs in a range which makes the inert anode economically viable for commercial application.
To implement the above-stated objects the instant invention of an inert anode for electrolytic aluminum production has been devised.
The anode is of modular construction consisting of a plurality of parallel vertical wires mounted on horizontal support structure which may be: (1) linear and extensible to fit large furnaces, singly or in parallel; or (2) circular, singly or in multiple concentric circles. This geometry provides a high surface-to-volume ratio which supports efficient electrolytic action. The connection to an electric power supply is through the support structure.
The support structure and the wires, typically xe2x85x9 inch in diameter, are made of a high-temperature corrosion-resistant metal alloy such as ASTM A297, ASTM A351, or AISI 330. These alloys are not attacked by fused salts or molten metals at the elevated temperature of the electrolytic bath.
The wires are completely surface-coated with a noble metal such as platinum to a thickness in the range of 1 to 10 microns. A durable noble-metal coating process such as the proven SCX sputter coating process or equivalent is used to attach the coating permanently to the wires.
The melting points of the metal alloy and the platinum are considerably above the bath temperature to ensure that the anode wires and manifolds remain in the solid state and structurally strong at all times. The corrosion-resisting and catalytic properties of the platinum ensure that the anode surfaces do not corrode, are not consumed, and able to dissociate any oxides formed in the process. Also, bare spots due to inadvertent handling nicks, bruises or abrasions are of no consequence for continuous electrolysis operation since the metal alloy base material is heat-resistant and also resists corrosion by molten fluoride salts.
The electrical conductivity of the metal wire anodes is of the order of four times higher than that of carbon, thus reducing the power input to the furnace, typically by a factor of one-half, compared to carbon anodes.
The physico-chemical characteristics of the inert anode of the invention described above give rise to the following economic and environmental advantages for electrolytic aluminum production as a whole:
1. Cost reduction due to lower electrical power requirement;
2. Higher productivity due to enhanced electrocatalytic action:
3. Environmentally clean industry due to zero emission of greenhouse gases, including perfluorocarbon gases;
4. Capital cost savings due to shutdown of carbon-making plants for anodes, even when offset by cost of replacement alloy and platinum anodes;
5. Higher quality aluminum metal due to reduced contaminants in extraction process; and
6. Application to electrolytic furnaces of variable size due to modular nature of anodes permitting linear or concentric expansion of anode surface.